Radiant energy is used in a variety of manufacturing processes to treat surfaces, films, and coatings applied to a wide range of materials. Specific processes include but are not limited to, curing (i.e., fixing, polymerization), oxidation, purification, and disinfection. Processes using radiant energy to polymerize or effect a desired chemical change is rapid and often less expensive in comparison to a thermal treatment. The radiation can also be localized to control surface processes and permit preferential curing only where the radiation is applied. Curing can also be localized within the coating or thin film to interfacial regions or in the bulk of the coating or thin film. Control of the curing process is achieved through selection of the radiation source type, physical properties (for example, spectral characteristics), spatial and temporal variation of the radiation, and curing chemistry (for example, coating composition).
A variety of radiation sources are used for curing, fixing, polymerization, oxidation, purification, or disinfections due to a variety of applications. Examples of such sources include but are not limited to, photon, electron, or ion beam sources. Typical photon sources include but are not limited to, arc lamps, incandescent lamps, electrodeless lamps, and a variety of electronic (i.e., lasers) and solid-state sources. Conventional arc type UV lamp systems and microwave-driven UV lamp systems use tubular bulb envelopes made of fused quartz glass or fused silica.
FIG. 1A is a perspective view of a UV curing lamp assembly 10 showing an irradiator 12 and a light shield assembly 14 in the prior art. FIG. 1B is a partial cross-sectional view of the lamp assembly 10 of FIG. 1A showing a half-elliptical primary reflector 16 and a light source 20 of circular cross-section. FIG. 1C is a partial cross-sectional internal view of the light shield assembly 14 of FIG. 1A showing a half-elliptical primary reflector 16 and a light source 20 of circular cross-section mated to a secondary reflector 25 and end reflectors 26.
Referring now to FIGS. 1A-1C, the UV curing lamp assembly 10 includes an irradiator 12 and a light shield assembly 14. The irradiator 12 includes a primary reflector 16 having a generally smooth half-elliptical shape studded with a pair of RF slot openings 18 for receiving microwave radiation to excite a light source 20 (to be discussed hereinbelow), and a plurality of openings 22 for receiving air flow to cool the light source 20. The light source 20 includes a lamp (e.g., a modular lamp, such as a microwave-powered lamp having a microwave-powered bulb (e.g., tubular bulb with a generally circular cross-section) with no electrodes or glass-to-metal seals). The light source 20 is placed at the internal focus of the half-ellipse formed by the primary reflector 16. The light source 20 and the primary reflector 16 extend linearly along an axis in a direction moving out of the page (not shown). A pair of end reflectors 24 (one shown) terminate opposing sides of the primary reflector 16 to form a substantially half-elliptical reflective cylinder. The light shield assembly 14 of FIG. 1A-1C includes a secondary reflector 25 having a substantially smooth elliptical shape. A second pair of end reflectors 26 (one shown) terminates opposing sides of the secondary reflector 25 to form a substantially half-elliptical reflective cylinder.
A work piece tube 28 of circular cross-section is received in circular openings 30 in the end reflectors 26. The center of the openings 30 and the axis of the work piece tube 28 are typically located at the external focus of the half-ellipse formed by the primary reflector 16 (i.e., the internal focus of the half-ellipse formed by the secondary reflector 25). The work piece tube 28 and the secondary reflector 25 extend linearly along an axis in a direction moving out of the page (not shown).
FIG. 2A is a perspective view of a tubular light source 20 having a generally circular cross-section in the prior art for use with the UV curing lamp assembly 10 of FIG. 1A-1C. FIG. 2B is a perspective view of a primary reflector 16 with the tubular light source 20 of FIG. 2A inserted therein, the primary reflector 16 having openings for receiving microwave radiation to excite the light source 20 and openings for receiving air flow to cool the light source 20 for use with the UV curing lamp assembly of FIG. 1A-1C. Referring now to FIGS. 1A-2B, the light source 20 (e.g., an electrodeless bulb 20 or arc lamp 20) has a pair of end sections 31 and a center section 32 that has a tapered shape, the end sections 31 and the center section 32 each having generally circular cross-section. The light source 20 is filled with a gas. The light source 20 has a pair of short quartz stubs 34 of having a substantially circular cross-section at either end to provide mechanical support for quick mounting into spring-loaded receptacles (holes) 36 located in the end reflectors 24. These stubs 34 are not electrodes and have no electrical function. Arc lamps are energized through electrodes at each end.
The light source 20 is placed at the internal focus of the half-ellipse formed by the primary reflector 16. The light source 20 and the primary reflector 16 extend linearly along an axis in a direction moving out of the page (not shown). A pair of end reflectors (not shown) terminates opposing sides of the primary reflector 16 to form a substantially half-elliptical reflective cylinder, and have slots (not shown) configured for receiving the stubs 34 of light source 20.
In operation, gas in the light source 20 is excited to a plasma state by a source of radio frequency (RF) radiation, such as a magnetron (not shown) located in the irradiator 12. The atoms of the excited gas in the light source 20 return to a lower energy state, thereby emitting ultraviolet light (UV). Ultraviolet light rays 38 radiate from the light source 20 in all directions, striking the inner surfaces of the primary reflector 16, the secondary reflector 25, and the end reflectors 24, 30. Most of the ultraviolet light rays 38 are reflected toward the central axis of the work piece tube 28. The light source 20 and reflector design are optimized to produce the maximum peak light intensity (lamp irradiance) at the surface of a work product (also propagating linearly out of the page) placed inside the work piece tube 28.
When the plasma in the light source 20 is excited and produces UV radiation, the surface of the light source 20 becomes very warm. Cooling air enters a reflector cavity 40 formed by the primary reflector 16, the secondary reflector 25, and the end reflectors 24, 30 through the pair of RF slot openings 18 and the plurality of openings 22 in the primary reflector 16 and flows across the light source 20 at sufficient volume to maintain the light source 20 at its optimum temperature. Sufficient air must be drawn through the reflector cavity 40 to maintain the bulb envelope temperature below a critical temperature of 900-1000° C. In arc lamps, the electrode seals must be maintained at an even lower temperature. At higher temperatures, the lifetime of the light source 20 may be reduced. UV output power for both microwave-powered lamp systems and arc-driven UV lamp systems is limited only by how much cooling can be provided to the light source 20. UV lamps that operate at higher power levels are more desirable, since they can cure a work product (e.g., coatings) at a faster rate, making them more productive.
Either an integral blower (mounted on the irradiator 12) or a remote blower may be used to provide cooling air. It is desirable to reduce the amount of cooling air needed to sufficiently cool the light source 20. As a result, the blower speed or the blower size may be reduced as well. For certain environments, a lower blower speed or smaller blower size advantageous, since such a blower outputs a lower noise level.
The optics generally used in UV systems incur compromises relating to the diameter of the light source 20. Larger bulb diameters may be operated at higher power levels because they have more surface area and therefore require less cooling for a given power input. However, the collection efficiency of reflective optics is not as high with larger diameter bulbs. When elliptical reflectors are used to collect and focus UV radiation from the light source 20 onto a work product, the higher the collection efficiency and the higher the peak irradiance developed at a working plane which includes the work product, the faster the work product may be cured.
Unfortunately, not only do larger bulbs not focus to as high an irradiance level due to divergence, they also block a bit more of the reflected UV radiation from the apex 40 of the ellipse formed by primary reflector 16 due to their larger diameter. Some of the UV radiation that is directed back at the light source 20 becomes trapped in the plasma and does not contribute to the UV output of the light source 20.
As discussed above, current electrodeless bulbs that emit ultraviolet radiation for curing work pieces have an elongated cylindrical shape of circular cross-section. When the light source 20 containing a gas is excited with microwave radiation, a plasma develops which causes the surface of the bulb to heat up to high temperatures. The bulb is generally air cooled through the primary reflector 16 on one side of the light source 20, which causes the other side of the light source 20 to not receive proper cooling. This causes the light source 20 to develop hot spots which reduces the life of the bulb.
The aforementioned problems with cooling result from the shape of the light source 20 and the size and location of the RF slot openings 18 and the plurality of openings 22 of the primary reflector 16.
FIG. 3 shows velocity profiles of air flow across the length of the light source 20 of the prior art and the primary reflector 16 for different levels of air velocity. FIG. 4A shows velocity profiles of air flow normal to the light source 20 of the prior art in the vicinity of the RF slot openings 18 of the primary reflector 16. FIG. 4B shows velocity profiles of air flow normal to the light source 20 of the prior art in the vicinity of the smaller openings 22 of the primary reflector 16. FIG. 5A shows surface flow wrapping of air around the light source 20 of the prior art in the vicinity of the RF slot openings 18 of the primary reflector 16. FIG. 5B shows surface flow of air diverging near the side of the light source 20 of the prior art distal to the smaller openings 22 of the primary reflector 16. Referring now to FIGS. 3-5B, the flow of air differs along the length of the light source 20, with greater levels of air flow near the RF slot openings 18 and lower levels of air flow therebetween emanating from the plurality of openings 22. Thus, in the regions 40 of the light source 20 near the RF slot openings 18, the air flow pattern 42 envelopes the light source 20, thereby lowering the temperature of the light source 20 effectively. In other regions 44 of the light source 20 near the plurality of openings 22, the air flow pattern 46 bows out, wherein it flows across the light source 20 on the side 48 nearest the apex 40 of the primary reflector 16, but is absent on the side of 50 of the light source 20 distal to the apex 40 of the primary reflector 16, thereby causing a significant increase in temperature relative to the temperature of the light source 20 proximal to the RF slot openings 18 as depicted in FIGS. 6A-6C to be discussed hereinbelow.
FIG. 6A shows a top down view of a light source 20 of the prior art overlying a primary reflector 16 with gray scale shading along the light source 20 indicating relative temperature. FIG. 6B shows a perspective view of a light source 20 of the prior art and the primary reflector 16 of FIG. 6A with grey scale shading along the light source 20 indicating relative temperature, and direct indications of the temperature of the lamp proximal to the RF slot openings 18 of the primary reflector 16 and near the center of the light source 20. FIG. 6C is a plot of temperature versus distance along the light sources 20 of FIGS. 6A and 6B. Referring now to FIG. 6A-6C, the hottest spots 51 on the light source 20 are shifted slightly to the interior of the RF slot openings 18, and having a temperature of about 1012° C., represented by a lighter shade of grey. The coolest spots 52, 54, represented by deeper shades of grey, may be found in the immediate vicinity of the RF slot openings 18 and near the center of the light source 20, respectively. From FIGS. 3-6C, it would be tempting to increase the size of the plurality of openings 22 to increase airflow around the light source 20. However, a person skilled in the art would appreciate that this may result in an increase of UV radiation escaping though plurality of openings 22, thereby reducing the peak UV curing irradiance of the work product.
Accordingly, what would be desirable, but has not yet been provided, is a light source having lower cooling requirements and that provides increased peak UV curing irradiance.